The field of electronically conductive polymers has advanced rapidly during the last 20 years with numerous examples of prior art. While most organic polymers exhibit negligible electronic conductivity of less than 10.sup.-12 reciprocal ohm centimeters (i.e., Siemans/cm), some polymers exhibit electrical conductivity or semiconductivity, however. Conductivity values for such polymers have been demonstrated from 10.sup.-10 to 10.sup.6 S/cm, covering the range associated with semiconductors and conductive filled composites, at the lower end, to metals at the higher end.
Polymers with intrinsic electrical conductivity or semiconductivity are of interest as lightweight replacements for inorganic metals and semiconductors. Of further significance is that they may be processible at temperatures normally associated with polymers, that is about 100-400 degrees centigrade. In addition, conductive polymers have the prospect to replace metal coatings, but which may be applied by economical non-vacuum processes such as from solution or from the melt. Furthermore, organic conductive and semiconductive polymers may be synthetically tailored to optimize desirable properties such as melting point, melt viscosity, solubility, electrical and thermal conductivity, optical and microwave reflectance and absorbance, electroluminescence, electrochromism, and linear and nonlinear optical refractive index. Semiconducting polymers with conductivity on the low end of the range but with bandgaps corresponding to visible wavelengths have been used in light emitting diode (LED) and laser structures.
In general, conductive/semiconductive polymers consist of a backbone of repeating monomer units with extended pi electron delocalization. Examples of backbones that are popular for forming conductive or semiconductive polymers are: ##STR1##
Prior art describes many variations of these simple backbones, for example materials which are monodisperse (i.e., all polymer chains are the same) or which have different monomer substitution patterns to enhance selected chemical, mechanical, optical or electrical properties. These are only examples of prior art, as numerous other repeated pi conjugated systems have been described as having electrically conductive properties. Favorable electrically conductive properties are frequently observed not only in polymers with hundreds or thousands of repeating units, but also in oligomers with 4 to 10 repeating units.
Conductive/semiconductive polymers may be employed either in their unmodified state or they may be "doped" to enhance their electrical conductivity. Doping a conductive polymer entails chemically treating the backbone to produce mobile charge carriers. Frequently this involves partial oxidation or reduction of the chain. In the case of polythiophene, for example, oxidation of the chain by exposure to oxidants such as I.sub.2 results in an oxidation-reduction reaction which produces mobile positive charges on the backbone and charge compensating anions such as I.sup.-, the latter being retained in the solid polymer matrix. Other conductive polymers, such as polyacetylenes, may be doped by treatment either with reducing agents or with oxidizing agents. Still another class of conductive polymers such as the polyanilines may be doped by protonation with acids rather than by oxidation-reduction, such protonations leading to structural rearrangements that produce mobile positive charges on the extended pi backbone.
It is evident from the examples that conductive polymers of prior art are essentially linear chain oligomers and polymers. When these polymers are aggregated in the solid state overall charge transport is thought to depend both on the mobility of the charge along the backbone and, particularly, on the ability of the charge to hop from chain to chain. This intermolecular charge transfer will be enhanced by short intermolecular separation distances and by overlap between the pi systems. Structural disorder is to be avoided because charge can become trapped within inhomogeneous regions of the solid matrix where intermolecular separation is large and/or pi overlap is small. Therefore, the conductive or semiconductive properties of a conductive polymer will be enhanced by long range ordering of the polymers in the solid state. Polymer structures are therefore desired which promote such long range ordering and also intermolecular spacings and pi overlap which favor interchain charge hopping.
An example where structural manipulation has been used in prior art to enhance the solid state structural order and the resulting conductivity is in the preparation of so-called poly(3-alkylthiophenes) or PATs that are regioregular (R. D. McCullough et al., Journal of the American Chemical Society 113, 4910 (1993) and references cited therein; T. Chen, R. D. Rieke, J. Am. Chem. Soc. 114, 10087 (1992)). When the PATs are synthesized by standard polymerization reactions, the polymer is a mixture of the three linking patterns shown below: ##STR2## where R represents the alkyl group. Synthetic methods have been developed so that the rings are connected exclusively in the head-to-tail (h-t) fashion, and these "regioregular" polymers are referred to as "rPATs". In the randomly coupled PATs the interaction between the alkyl side chains leads to structural disorder and reduces the length of the structures in the solid state over which charge can travel before being trapped or retarded by defects. By contrast, thin films cast with the rPATs are known to self-assemble to give a highly ordered structure. The rPAT films cast from solution give a sharp x-ray spectrum while no x-ray spectrum is observed for the randomly structured analogs. The visible spectral absorption maximum both in solution and the solid state occur at a longer wavelength (lower energy) for the rPATs indicating a greater effective conjugation length through intermolecular interactions of the polymer chain compared with the irregular PATs. The electrical conductivity of the rPAT films is 2 orders of magnitude higher than films made with the nonregiospecific PATs. X-ray and microscopic analysis indicate that the PATs assemble in two dimensional sheets, which is manifested as a laminar morphology in thin films cast from solution. As a result, the electrical conductivity is higher in the plane of the sheets (parallel to the substrate) than it is through the sheets (perpendicular to the substrate). The laminar morphology produces a rough texture to films cast from rPATs.
Regioregularity may apply to other polymer chains, for example 3-substituted polypyrroles and to 1,4 polythiophenes.
In another example of prior art, an electrically conducting dendrimer has been reported by R. G. Duan, L. L. Miller, and D. A. Tomalia, J. Am. Chem. Soc. 117, 10783 (1995). Films 3-5 .mu.m thick of a poly(amidoamine) dendrimer with naphthalene diimide anion radicals on its periphery were cast from formamide. The electronic conductivity of the films was as high as 6.times.10.sup.-2 Siemans/cm. The conduction path is along the .pi.-stacks that were formed in contrast to the traditional conducting polymers where the conducting path is based on alternating single and double bonds p-conjugated organic system. Significantly, because the dendritic structure can form the p-stacks in three dimensions, the conductivity was isotropic rather than anisotropic as is usually observed in other conductive, layered systems. While demonstrating a principal of three dimensional conduction, these materials do not take advantage of the capabilities of existing conducting polymers or of the variety of conjugated organic structures that can be formed.
It is therefore an object of this invention to provide a new class of conductive and semiconductive polymers, defined herein as "star conductive polymers", that can self-assemble into three dimensional solids similar to most metals but which employ linear conducting polymer structural units.
It is a further object of the invention to provide conductive polymers that form highly smooth coatings when cast from solution because the components can assemble in three dimensions rather than two dimensions.
It is a further object of the invention to provide star conductive polymers which can be doped by oxidative, reductive or protonation processes already established for the linear conducting polymer units.
Other objects and advantages of the invention will become apparent from the description of the invention which follows, made with reference to the drawings.